The use of frequency discrimination to measure near carrier frequency modulation noise of a microwave signal is discussed by Ashley et al. in the IEEE Transactions on Microwave Theory and Techniques, Volume MTT-16, No. 9, September 1968, pages 753-760. Prior art discrimination at microwave frequencies, as described in this paper, depended on a microwave cavity as a resonant circuit. Prior art discrimination at very high frequencies (VHF) utilize such components as slope detectors and ratio detectors, depending on the lumped circuit elements of inductance and capacitance, to form resonant circuits. At ultrahigh frequencies (UHF) between the VHF and microwave region, there is little prior art on such discrimination means. Most measurements in this area are made by heterodyning the UHF signal with a local oscillator to obtain a VHF signal which is analyzed with a discriminator at the VHF frequency range. Accounting for frequency modulation (FM) noise of the local oscillator is a well established problem in these areas. Both the cavity method and the lumped circuit element method obtains a rejection of residual amplitude modulation (AM) noise on the signal under test; for example, the VHF discriminator usually employs a limiter or ratio detector. The microwave cavity discriminator rejects AM noise and also allows the input signal level to be increased, and provides the greatest discriminated output to improve the signal-to-threshold ratio, with the threshold being the lowest value of noise established or obtained within the analyzer system.
Among the discriminators discussed in prior art literature which might be useful in obtaining FM noise measurements at UHF or lower microwave frequencies is the transmission line discriminator as has been noted by R. A. Campbell in "Stability Measurement Techniques in the Frequency Domain", IEEE-NASA Symposium on Short Term Frequency Stability, NASA SP-80, November 23-24, 1964, pages 231-235. Various aspects of detection equipment and calibration are disclosed. Also, in the IEEE Transactions on Microwave Theory and Techniques, Volume MTT-23, No. 9, September 1975, pages 776-778, an article entitled "Single Hybrid Tee Frequency Discriminator" by J. Nigrin et al. discloses a discriminator tuned by a movable short and has properties comparable to those of a phase discriminator. Additional prior art includes U.S. Pat. No. 3,675,124 entitled "Apparatus for Measuring Frequency Modulation Noise Signals and for Calibrating Same" by J. R. Ashley et al. This apparatus employs an auxiliary injection phase-locked oscillator driven by a test oscillator and relies on a discriminator cavity resonator which must be accurately tuned to the exact operating frequency for acceptable operation. Ashley et al. discusses in column 6 the difficulty of making signal measurements where limited power outputs make prior art methods ineffective.